"Incredible" is part of the Hulk's moniker, but it can apply to Bruce Banner in a lot of different ways, too: incredible dork, incredible neurotic, even incredible fool. All the same, we don't want to judge him too harshly. It must be tough passing out every time you get cheesed off, then waking up to a destroyed city and a totally-ruined pair of pants.
So let's take a look at what's ailing our goth-y green giant.
A Capable Enabler
As the only other scientist in Avenger-land, Bruce Banner is on hand to help Tony Stark bring Ultron to life. To Banner's credit, though, he's not only smart enough to troubleshoot the technical details, he's also sharp enough to point out the ethical implications:
Tony Stark: I don’t want to hear the "man was not meant to meddle" medley. I see a suit of armor around the world.
Bruce Banner: It sounds like a cold world, Tony.
Tony Stark: I've seen colder.
Had he been assertive enough to press Tony for details, Banner might have found out that Tony's "colder" world was only the result of a Wanda Maximoff-induced hallucination. He's not the assertive type, though, and so Bruce allows Tony to steamroll right past his very valid criticisms, thus enabling Ultron into existence—bad move.
Oddly, Banner is right back by Stark's side when they try the same move for a second time, using J.A.R.V.I.S. and the android they've stolen—er, liberated—from Ultron. Perhaps the duo both cling to an unshakable belief in their abilities, or (rightly) see J.A.R.V.I.S. as the key to fixing what went wrong the first time. It could also just be a case of Tony refusing to learn from his mistakes and Bruce being unable, or unwilling, to stand up to him.
Luckily, everything works out. Still, it would be nice if Bruce made a fist every now and again (a human-sized fist, not a titanic green, smashy one).
Unfortunately, Bruce leaves all of the fist-making up to his alter-ego, the Hulk. He'd really rather there'd be no fists at all, though, thank you very much. His time spent as the green machine—and the resulting damage caused by the Hulk—leave Bruce stricken by guilt and anxiety. That's better, in his view, than anger, which can kick things off all over again. Instead, he lives his life in constant fear that he'll lose control.
As you can imagine, this level of repression puts a severe dent in his prospects for romantic fulfillment. Even though Black Widow is willing to look past his Hulk tantrums, he's not:
Black Widow: How long before you trust me?
Bruce Banner: It's not you I don't trust.
That's sad, Bruce. As much as he'd like to, he's incapable of letting down his guard for fear of what the Hulk will do.
In his defense, after he gets zapped by Wanda Maximoff, the Hulk does destroy the entire city of Johannesburg—not good. Once that happens, Bruce doubles down on the repression, refusing to Hulk out even when his friends really need him in the final battle for the floating Sokovia.
Finally, Black Widow catches him off guard with a kiss—and a loving shove off a tall ledge. Her advice?
Black Widow: Now go be a hero.
And that's exactly what the Hulk does. It seems that, as she directs his anger and soothes him back to human form, Black Widow really does have the ability to connect with the Hulk in a way that no one else can. It's the central tragedy of the movie that Bruce fails to realize his.
Instead, we see him moodily staring out of the Quinjet window as Hulk flies off into the sunset without his best girl. Hulk need buddy movie with Thor to sort out conflicted feelings of love.